Colt did tests on the M4 and M4A1 on full auto mag dumps without stopping to cool.

The M4 made it to around 540 rounds before the barrel drooped and burst.
The M4A1 made it to 910 or so before the rifle was on fire and unable to be used.

If you don't know the difference in the barrels google it. As for barrel life, Filthy 14 was a standard CL barrel that was getting 4-5MOA at 100yrds with 34,000 rounds through the barrel at rates of 1,300 every 3 days(this is considered a heavy firing schedule).

Its also woth noting that if you abuse a barrel by firing 10 or so mags without a break(this us abusive because no proper trainig can come from it) the throat will be gone by around 6,000-7,000 rounds.